1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for patterning a layer having a high reflectance using a photosensitive material film.
2. Description of the Prior Art
In the manufacture of semiconductor devices, a conductive layer such as an aluminum layer is patterned using a photosensitive material film in order to obtain a wiring layer. A positive- or negative-working photoresist is used as a photosensitive material film. As is well known, the portion of a positive-working photoresist which is exposed decomposes and can be removed by developing, thereby providing an etching mask used in etching a conductive layer. Conversely, the portion of a negative-working photoresist which is exposed is hardened and the portion which is not exposed can be removed by developing, thereby providing an etching mask.
A conventional method for etching an aluminum layer using a positive-working photoresist will be described with reference to FIG. 1. An aluminum layer 3 is formed by, for example, vapor deposition on a semiconductor wafer 1 through an oxide film 2. A positive-working photoresist film 4 is formed on the aluminum layer 3. A photomask 5 having a nontransparent region 5a and a transparent region 5b is arranged above this structure.
Collimated light 6 from a light source (not shown) is transmitted only through the transparent region 5b of the photomask 5 and exposes the photoresist film 4 in a predetermined pattern. If the oxide film 2 has an inclined surface 2a and the aluminum layer 3 correspondingly has an inclined surface 3a, light is reflected due to the high reflectance of aluminum. The transverse reflected light 6a exposes the portion of the photoresist film 4 which should not be exposed. When this happens, a desired etching mask pattern cannot be obtained after developing. When the aluminum layer 3 is etched using the photoresist film having such a mask pattern, an aluminum wiring layer 3' as shown in FIG. 2 is obtained. Referring to FIG. 2, an edge line 3'b of the aluminum wiring layer 3' close to the inclined surface 2a does not extend to a correct pattern edge line indicated by an alternating long-and-short dashed line 3'a. In the worst case, a disconnection 3'c may be formed. Needless to say, a semiconductor device having such an aluminum wiring layer 3' cannot perform its intended function.
On the other hand, when an aluminum wiring layer as described above is obtained using a negative-working photoresist, the above-mentioned problem is not encountered since the positional relationship between the transparent and nontransparent regions of the photomask used is the opposite of that described above. However, incident light for hardening a predetermined part of a negative-working photoresist film is reflected by the aluminum layer underlying the negative-working photoresist film. Then, the reflected light and the incident light cause interference in the negative-working photoresist film to generate intense standing waves, which leads to nonuniform exposure of the photoresist film in the direction of its thickness. The photoresist film is exposed only to weak light, especially in the vicinity of the surface of the aluminum layer. When such a photoresist film is developed, the portion thereof which ought to remain instead separates away.
This phenomenon associated with a negative-working photoresist is also experienced with a positive-working photoresist. In the latter case, the portion of the positive-working photoresist film which is to be removed remains unremoved, and the aluminum wiring layer may be short-circuited by any adjacent aluminum wiring layer. This short-circuiting will also occur when the positive-working photoresist 4 is replaced by a negative-working photoresist and the same photomask 5 is used as shown in FIG. 1.
The phenomena as described above are encountered not only in the patterning of aluminum layers but also in the patterning of other inorganic conductive material layers having a high reflectance.